During the previous funding cycles, our program demonstrated that donor-specific memory T cells generated during protective immune responses against environmental pathogens constitute an important barrier to tolerance induction in adults. More recently, within the context of the increasingly-appreciated heterogeneity in memory T cell populations, a modern view of alloimmunity is developing, recognizing that a patient's risk for rejection and their susceptibilities to tolerance induction are highly individualized and determined in large part by their cumulative history of episodic and sustained encounters with a diverse array of environmental pathogens. The unifying hypothesis and theme of this program is that the efficacy of tolerance induction in a given individual is a function of the specific type and frequency of environmental pathogen exposure, in that these exposures generate distinct memory profiles which are composed of populations of highly specialized subsets of variable size. Accordingly, this program will be comprised of three inter-related projects. Project I seeks to define the precise mechanisms by which tolerance can be induced in na[unreadable]ve T cells by CD28 and/or CD40 blockade, and then to define specific aspects and subsets of the memory compartment that pose the largest barrier to tolerance, including traditional CD4+ and CD8+ TCM and TEM as well as the newly described, potentially very interesting subsets of CD4+ Th1 memory cells that have been shown to have different phenotypic, functional, and homing properties. Analysis of these subsets will be the major focus of Project II, which will pursue in-depth studies of the differentiation and lineage relationships of these subsets, and will examine their relative abilities to provide help for protective B and T cell responses. Project III will test the hypothesis that the immune control of latent herpesvirus infections fosters, through the generation of viral-specific memory T cells, the maintenance of memory phenotype T cells with heterologous alloreactivity. These cells may be refractory to immunosuppression in general and to T cell depletion in particular, and to the extent that they cross-react with donor antigens, they may specifically prevent allograft acceptance. The knowledge gained through these studies will facilitate development of tolerance induction protocols that are efficacious, safe, clinically applicable, and have the potential to be tailored to fit the environmental pathogen exposure of each individual patient in order to maximize graft survival. PROJECT 1: Determinants of T Cell Fate (Larsen, C) PROJECT 1 DESCRIPTION (provided by applicant): Transplantation is the preferred mode of therapy for many forms of end-stage organ disease. Success in transplantation has been built upon therapeutic approaches to control the T cell-dependent process of rejection. Thus, a unifying goal of tolerance induction strategies is to selectively inactivate or delete donor-reactive T cells. Transient blockade of the CD28/B7 and CD154/CD40 pathways has shown great promise in tolerance protocols in na[unreadable]ve recipients, particularly those that induce mixed hematopoietic chimerism and robust donor-specific tolerance. Unfortunately, the anti-CD 154 mAbs that are a cornerstone of this approach cause platelet dysfunction and thromboembolism. In addition, the presence of donor-specific memory T cells as a result of heterologous immunity present a potent barrier to transplantation tolerance. Thus, these two factors present challenges that must be met if costimulation blockade to induce transplantation tolerance is to become a clinical reality. Despite years of interest in these pathways, our knowledge of the mechanisms by which CD28 and CD154 blockade synergize to promote the deletion of na[unreadable]ve donor-reactive T cells remains very incomplete. By extending our knowledge of this process, we may identify new opportunities to program na[unreadable]ve donor-specific T cells to execute a death program rather than an expansion and differentiation path that leads to rejection. While there has been considerable progress in defining the multiple mechanisms by which CD40 transduces signals, at present we have little insight into which of the CD40-associated adaptor molecules and signaling pathways must be interrupted to promote deletion of na[unreadable]ve donor-reactive T cells. Furthermore, it is now well-established that memory T cells are less susceptible to the effects of CD28 and CD40 blockade. Thus, the immune history of a transplant recipient and levels of donor-cross-reactive memory T cells within the various compartments (CD4+ or CD8+ TEM or TCM) may dictate the likelihood of success or failure of attempts at tolerance induction or even immunosuppression. By understanding the functions, costimulatory and signaling requirements for recall responses mediated by the various memory T cell subsets, we may be able to tailor tolerance induction approaches to control the predominant forms of memory for specific donor-recipient combinations. Lay Summary: Transplantation represents a cure for many terminal diseases. However, transplant recipients require lifelong immunosuppression to prevent immunological rejection of the allograft. The goal of this grant proposal is to understand the signals that immune cells (T cells) require to reject transplants. With this knowledge we will design methods to control these cells to develop methods of inducing long-lasting transplant acceptance without the need for toxic immunosuppressive regimens.